A blog from a science nut, computer geek, sometimes philosopher, and a human. Based out of Denver, with a love of Antarctica, I seem to wander the world so who knows where I'll be in 6 months.

Antimicrobial vs Waterless Handwashing

by wonderfullyrich on June 2, 2008

I was revisiting the topic of soap today as I was thinking about the continuing discussion that i’ve had where people argue hand sanitizer are as bad for the Environment as anti-microbial soap and decide that it was worth continuing (and perhaps ending) this conversation.

Many of you have likely heard about the superbug or a anti-bacterial resistant bacteria which cannot be killed with current drugs, and one aspect of this is the anti-microbial soap. I don’t want to go over everything on how it works (Here’s a good article on how soap, anti-microbiocial soap, and waterless agents work to clean, in addition to my post.) The key point though is this, antibiotic resistance is fostered by residual effects of washing with anti-microbial soap AFTER you’ve washed it down the drain. Anti-microbial soap is a mechanical and chemical reaction which lifts and kills then drains the dead bacteria, soap, and anti-microbials into the watershed. A report on how we still see anti-microbial products from 50+ years ago in groundwater. Waterless washing, aka hand sanitizers like Purell, with alcohol that is above %60 will kill of the bacterial because it’s a solvent. Which means it able to breaks through the bacteria’s cell wall effectively disintegrating the bacteria from the inside, but after a few mins it will then evaporate, giving the bacteria no time to settle in the water and evolve a resistance. I modified this graphic on wikipedia’s entry on antibiotic resistance to illustrate the point.

I think what confuses people about this is how evolution works. This is a scale issue, as one a small scale, i.e. your hands the number of bacteria that are likely to have randomly evolved an immunity to anti-microbial soap is infinitesimal, however once you introduce it into the ground water you’ve increase the scale and pool of candidates and therefore the likelihood you’ll eventually produce a bacteria that is resistant to the anti-microbial and renders it ineffective. More over by continuously introducing anti-microbial solutions into the ground water, you’ve created an environmental niche where anti-microbial resistant bacteria are the only ones who will thrive and there for are likely to spread.

Waterless agents are disconnected from this evolutionary problem given that once they’ve killed that small scale area, i.e. your hands, then it evaporates into the air, where it becomes another gaseous molecules. (Air does have bacteria floating around in it, but the density difference between gas and liquid dramatically reduces probabilities of such evolutionary niches being built–from say 1 in 100 to 1 in a billion, or a similar magnitude.) Waterless agents don’t leak into the groundwater environment and therefore give the bacteria on the larger scale less chance to evolve resistance.

Of course this not to say that waterless agent resistant bacteria won’t evolve, they can and might become resistant to the solvent powers of alcohol, (actually many are, however they are harmful to humans or cannot live in humans) but their resistance would have to come about in a more random fashion then by flourishing because it’s the most fit.

Hopefully this clears up the reason to use hand sanitizers and evaporative based cleaners rather then anti-microbial soaps of any sort.